The present invention relates to a process and a plant for cleaning hot waste gas produced in varying amounts.
It is conventional to clean hot waste gases, such as flue gases from power stations, incineration plants and heating stations to remove harmful components such as sulphur oxides, hydrogen chloride and nitrogen oxides to avoid environmental damages when the waste gas is released into the atmosphere.
A method which has proved most acceptable for removing said harmful components from hot waste gases is the so-called spray drying absorption process in which an aqueous solution or suspension of an absorbent for the harmful components is atomized into a stream of the hot waste gas to be cleaned. Thereby a substantial part of the harmful components is absorbed and the atomized solution or suspension of absorbent is dried forming a powder comprising the harmful components removed from the waste gas.
Said type of processes has hitherto mainly been applied for absorbing acidic components such as sulphur oxides and hydrogen chloride by means of basic absorbents such as hydroxide and carbonates of alkali metals or alkaline earth metals. A typical example of a process of said type is disclosed in U.S. Pat. No. 4,279,873.
In one of the preferred embodiments described in said U.S. specification the process is performed in a spray drying chamber having a cylindrical upper portion and a conical bottom portion. An atomizer for atomizing the aqueous absorbent is provided for in the ceiling of said chamber and hot waste gas is introduced into the chamber through a gas disperser surrounding the atomizer. Another stream of waste gas to be cleaned is introduced through a gas disperser arranged centrally in the chamber below the atomizer and blowing waste gas upwards against droplets of aqueous absorbent leaving the atomizer.
Gas dispersers suitable for spray drying absorption processes of the type described require a supply of waste gas within certain limits to be able to deliver a waste gas stream of suitable velocity to contact the atomized absorbent in a manner which ensures efficient operation. This is i.a. due to the fact that specific requirements exist as to velocity and direction of the streams of waste gas meeting the atomized droplets of absorbent in order to provide efficient contact of the gas with the droplets and at the same time create a gas flow pattern that prevents wet particles from getting into contact with and form deposits on the chamber wall. It is also required that the pressure drop over the gas disperser, and consequently the power consumption, is as low as possible.
Basically the principles used in spray drying absorption processes of the type described (e.g. desulfurization processes) allow substantial variations in the amounts of waste gas to be treated since the amount and concentration of aqueous absorbent may be adjusted within wide limits, especially when rotary atomizers are used. That means that the limits as to acceptable variations in the amount of waste gas to be treated will often be set by the characteristics of the gas disperser(s).
In large plants problems due to variations in the amount of waste gas produced may be solved by using a plurality of spray drying absorbers, which are put into operation or disconnected as required. However, in plants having only a few or a single spray drying absorber that possibility is not suitable or possible.
It has also been suggested to cope with said variations by using a gas disperser having adjustable cross section of the flow path. However, the means enabling such an adjustment are quite complicated and expensive and may be too delicate to ensure troublefree operation in connection with corrosive and ash containing waste gases such as flue gas. Gas dispersers with adjustable gas passage are described in e.g. published Danish patent application No. 5029/80.
In spray absorption processes for flue gas cleaning it has been suggested to promote the reaction between the atomized absorbent and noxious components of the flue gas by maintaining a high relative humidity in the central area of the drying chamber. This is made possible by injecting flue gas through apertures in the walls of the drying chamber to sweep said walls and thereby avoiding deposits of moist spent absorbent. Such a system is described in published Danish patent application No. 146,214.
It should be emphasised that the above mentioned process using an introduction of gas through apertures in the walls of the drying-absorption chamber to sweep said walls requires a fairly constant amount of gas to be introduced as wall sweep and if said amount drops under a certain volume moist spent absorbent will deposit on the walls and finally interrupt the operation.
Consequently said known process is not apt for treating very fluctuating amounts of waste gas.